There is often a need to measure near-acoustic vibrations, such as seismic vibrations, in harsh environments. Traditional seismometers involve the suspension of a reference or proof mass within a frame and a sensor to measure the relative motion between the frame and the proof mass as seismic waves act on the frame to move it. In this manner, traditional seismometers require the use of a proof mass. Use of a proof mass limits the robustness of the seismometer, making seismometers not well suited to harsh environments.
Optical sensors are well suited to such rugged environments because they have passive detective elements that are immune from magnetic noise and electrolytic corrosion. One well-established method of sensing is remote signal processing of fiber Bragg gratings. A fiber Bragg grating includes a distributed Bragg reflector constructed in a segment of optical fiber. It allows for the reflection of particular wavelengths of light while transmitting all others. The Bragg wavelength of the fiber is sensitive to strain, whereby strain on the fiber Bragg grating causes a shift in the Bragg wavelength. Accordingly, fiber Bragg gratings can be used as direct sensing elements for environmental measurands such as strain and temperature. That is, by measuring the shift in the Bragg wavelength, a strain applied to fiber may be measured. Thus, fiber Bragg gratings may be used in sensors, wherein a mechanism that is sensitive to environmental change is constructed so that energy may be translated into the distortion of an optical grating so as to allow the resulting change in absorbed optical power to be interpreted.